The present applicant has previously proposed in a published document of Japanese Utility Model Application unexamined No. 59-127372 (Japanese Utility Model Application No. 58-22048) a brightness and contrast adjusting apparatus in which a pulse is inserted into a predetermined portion of a video signal and this pulse is used to adjust the brightness and contrast of a picture displayed on a cathode ray tube of a television receiver.
In this apparatus, a predetermined pulse is inserted into a horizontal blanking period of, for example, a video signal, this pulse is detected by a detecting circuit which is provided at the final stage section, this detected pulse is compared with a reference value and on the basis of the compared result, a gain of a gain control amplifier is adjusted. The contrast and the like of the cathode ray tube are adjusted by controlling the height of the pulse inserted.
According to this apparatus, without the influence of temperature drift of the circuit, etc., it is possible to carry out the adjustment with significantly high precision.
When this apparatus is used, it is possible to carry out the adjustment of, for example, the contrast and brightness.
FIG. 2 shows a brightness and contrast adjusting apparatus to which Japanese Utility Model Application unexamined No. 59-127372 is applied. Referring to FIG. 2, a video signal applied to a video signal input terminal 1 is supplied to a mixer 2 in which it is mixed with a contrast adjusting pulse from a reference signal input terminal 3 and a brightness adjusting pulse from a reference pulse input terminal 4. The mixed signal is supplied to a gain adjusting circuit 5. The output from the gain adjusting circuit 5 is supplied to a sample and hold circuit 6 in which it is sampled and then held by a sampling signal supplied from a timing signal input terminal 7 having the same timing as that of a pulse supplied to the reference signal input terminal 3. The potential sampled and held is supplied to a comparator 8 in which it is compared with a reference value a from a reference signal input terminal 9. The gain of the gain adjusting circuit 5 is controlled by the compared output therefrom.
Thus, contrast of a picture displayed on a cathode ray tube is adjusted.
On the other hand, the signal from the gain adjusting circuit 5 is supplied to a video output amplifier 10. This video output is supplied to a cathode ray tube 11 and the magnitude of a beam current is detected by the video output amplifier 10. The detected signal is supplied to a sample and hold circuit 12 in which it is sampled and then held by a sampling signal supplied from a timing signal input terminal 13 and having a same timing as that of the pulse which is supplied to the reference signal input terminal 4. The potential sampled and held is compared in a comparator 14 with a reference value b from a reference signal input terminal 15. The beam current of the cathode ray tube 11 is controlled by this compared output. While as the beam current detecting circuit of the video output amplifier 10, various circuits may be considered, it is possible to use a circuit which is disclosed in U.S. Pat. No. 4,356,508, "Brightness Adjusting Circuit for a Cathode Ray Tube" that the present applicant had proposed.
When the brightness is adjusted by controlling the beam current of this cathode ray tube 11, it may be possible to adjust a grid potential and a cathode potential, too. Moreover, it may be also possible that in response to the detected result of the beam current, the pedestal level of the video signal is controlled to become a predetermined level.
In consequence, the brightness of the picture image displayed on the cathode ray tube 11 can be adjusted.
In this apparatus, however, if the contrast is adjusted by the gain adjusting circuit 5, it is natural that the magnitude of the brightness adjusting pulse in its output signal should be varied. As a result, when the contrast is adjusted, the brightness is also varied so that a troublesome operation such as to re-adjust the brightness after the contrast was adjusted or the like must be carried out.